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J. Phys. Chem. A 2006, 110, 3132-3137
Reaction of Anthracene with CH Radicals: An Experimental Study of the Kinetics between 58 and 470 K† Fabien Goulay,*,‡ Christiane Rebrion-Rowe, Ludovic Biennier, Se´ bastien D. Le Picard, Andre´ Canosa, and Bertrand R. Rowe Laboratoire PALMS, Equipe Astrochimie Expe´ rimentale, UMR 6627 CNRS-UniVersite´ , UniVersite´ de RENNES I, Bat. 11C, Campus de Beaulieu, 35042 Rennes Cedex, France
The rate coefficient of the reaction of the methylidine radical CH with anthracene has been studied over a wide temperature range (58-470 K) in a dedicated “Cine´tique de Re´action en Ecoulement Supersonique Uniforme” (Reaction Kinetics in Uniform Supersonic Flow) apparatus. The reaction exhibits a slight positive temperature dependence, which can be fitted to the expression k(T) ) (3.32 ( 1.00) × 10-10(T/298)(0.46(0.14) cm3 molecule-1 s-1. Even at the lowest temperature, the reaction remains very fast indicating that the kinetics are probably driven by a capture process.
1. Introduction During the past decade the understanding of the complex chemistry that occurs in the Universe has become a fascinating challenge for scientists. Cold and ultracold media, partly or totally shielded from harsh radiation fields, such as dense and diffuse interstellar clouds and planetary atmospheres of the outer solar system provide the most favorable environment for the appearance of this chemistry. Evidence for the presence of extraterrestrial complex organic molecules was found a long time ago in some meteorites,1 and today more than 125 circumstellar and interstellar molecules have been detected. Recently, the European Space Agency’s Huygens probe descended through the cloudy atmosphere of Saturn’s main moon, Titan, and will soon furnish the first direct measurement of chemical species in extraterrestrial atmospheres.2 Among this wealth of chemical species, polycyclic aromatic hydrocarbons (PAHs) are suspected to have a significant place in the chemical scheme of these cold media. In their neutral or ionic forms, PAHs have been detected3 in meteorites and thought to be present in interstellar clouds, where PAH cations could be good candidates as the carriers of the diffuse interstellar bands.4,5 In planetary atmospheres, these large molecules are cited by Wilson et al.6 as one of the ways for production of Titan’s haze, which hide the moon’s surface from direct Earth observations. The presence of PAHs in Titan’s atmosphere has been discussed by Sagan et al.7 and more recently by Trainer et al.8 At low temperatures (interstellar medium, planetary atmosphere) similar to high temperatures (circumstellar envelopes) the formation of PAHs is predicted to involve the chemistry of benzene9,10 (C6H6). Benzene is the first aromatic compound that has been detected by ISO (Infrared Space Observatory) in carbon-rich stars11 and also in Jupiter and Saturn.12 In recent observations Coustenis et al.13 have found evidence for C6H6 on Saturn’s main moon and pointed out the significant role of aromatic compound for gas-phase chemistry at low temperature. Despite the importance of aromatic com†
Part of the special issue “Ju¨rgen Troe Festschrift”. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Present address: Departments of Chemistry and Physics, and Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720-1460.
pounds, there are only few laboratory studies of reactions with benzene or PAHs in their neutral form14-16 and only one direct measurement of a radical-PAH reaction has been performed at low temperature.17 To understand the basic chemistry of such species, it is necessary to study experimentally and theoretically elementary reactions of PAHs with radicals involved in the chemistry of interstellar media or planetary atmospheres. In the atmosphere of outer planets and their satellites, small carbon-containing radicals such as methylidine are produced by methane photolysis, mainly by the Lyman-R solar flux. It follows that the significance of the radical in these atmospheres is directly linked to the quantum yield for its formation in the photodissociation of methane at 121.6 nm. However, recent studies have not been able to resolve the issue of methane photolytic yields at this wavelength. The measurements of Rebbert and Ausloos18 indicated a rather small value, close to 0.08. Mordaunt et al.19 suggested a larger value of about 0.5. More recently Romani20 and Smith and Raulin21 gave values of 0.1 and 0.06, respectively. To investigate the significance of these quantum yield schemes on the hydrocarbon chemistry of Titan’s atmosphere, Wilson and Atreya22 have used a onedimensional photochemical model. They showed that the methane photolysis quantum yield does not affect the abundance of important hydrocarbons such as acetylene (C2H2) or ethylene (C2H4). Only minor three-carbon-atom molecules are sensitive to the choice of the methane photolysis quantum yield. The nominal value seems to be the Romani’s value of 0.1. Previously, Canosa et al.23 argued that a low value would imply a barrier to the formation of CH + H2 in the unimolecular decay of the superexcited CH3 fragments which are formed in photolysis of CH4, which is unlikely on the basis of both ab initio calculations24 and kinetic results for the reaction and relaxation of CH(V ) 0) and CH(V ) 1) in collisions with H2 and D2 at ultralow temperatures.25 It should be noted that a large quantum yield for production of CH in the photolysis of CH4 had earlier been deduced from the experimental work of Braun et al.26 It follows that the methylidine radical could have a central place in the chemistry of several planetary atmospheres. It might also be noted that CH was one of the first molecules detected in the interstellar medium,27,28 in diffuse clouds where it plays a vital role in the gas-phase chemistry.29-31 In dense
10.1021/jp054537g CCC: $33.50 © 2006 American Chemical Society Published on Web 12/18/2005
Reaction of Anthracene with CH Radicals
J. Phys. Chem. A, Vol. 110, No. 9, 2006 3133
Figure 1. Schematic view of the CRESU apparatus.
clouds, although certainly of much less importance than the corresponding reactions with neutral and ionized atomic carbon, reactions of hydrocarbons with the methylidine radical can lead to the accretion of carbon to produce a larger carbon skeleton, opening the way to the formation of large organic molecules.32 The database on the kinetics of neutral-neutral reactions has been improved during the past decade due to the progress made in theoretical calculation and experimental measurements. Rate coefficients at low temperature have been measured using the well-known “Cine´tique de Re´action en Ecoulement Supersonique Uniforme” (CRESU, Reaction Kinetics in Uniform Supersonic Flow) facility at the University of Rennes I and Birmingham. The apparatus at the University of Rennes I is the only one capable of measuring the rate coefficient of reactions involving PAHs at temperatures relevant to the interstellar medium.17 In this paper, we present the result for the first rate coefficient measurement of the CH radical reaction with anthracene (C14H10) between 58 and 470 K. We have used a modified CRESU apparatus dedicated to the study of gas-phase chemistry involving condensable molecules.17 The measured rate coefficient is found to be close to the gas kinetic rate coefficient and shows a positive evolution with the temperature. The high value of the rate coefficient even at 58 K shows the importance of this kind of reaction for the chemistry of low temperature media. The knowledge of the nature of the products would also be of vital importance but will only be discussed below, since our experiment does not yet allow reaction products to be determined. 2. Experimental Section The results presented in this paper were obtained by using the modified CRESU apparatus that has been developed at the University of Rennes I. The basis of the technique and the modifications due to the use of PAHs have been described in detail elsewhere,17 and only a concise description will be given here. A general sketch of the apparatus is given in Figure 1. Low temperatures are achieved in the CRESU apparatus via the isentropic expansion of a buffer gas through a suitably designed Laval nozzle. The resulting supersonic flow is axially
and radially uniform in temperature, density, and velocity. The density of the flow (1016-1018 cm-3) is large enough to ensure local thermal equilibrium by collisions. To use low vapor pressure compounds, a special apparatus has been developed, in which the reservoir is heated at 470 K by circulating heated oil from a bath regulated in temperature (Lauda CS20CS) through the double-jacketed walls of the reservoir. The aluminum laval nozzle is embedded in a conical hollow aluminum support mounted on the reservoir and heated by thermal conduction. Several aluminum nozzles have been designed for this experiment, each providing a particular temperature and density for the selected carrier gas. It is also possible to generate a subsonic flow, whose temperature is the same as the temperature of the reservoir. All nozzles were characterized by impact-pressure (Pitot) measurements from which the Mach number, temperature, and density of the flow could be derived. Details of the flow characterization were discussed previously.17 Anthracene was provided by Sigma Aldrich at 99% purity and was used without further purification. It was vaporized upstream of the reservoir in an oven, whose temperature was maintained constant ((2.5 K) by a proportional integrated differential regulator in the 500-550 K range. The vapor pressure of anthracene at the temperature of the oven is modeled by Antoine’s law.17 Then the anthracene vapor is flushed into the nozzle reservoir by a carrier gas flow, at a typical rate of 500 SCCM (SCCM ) cubic centimeter per minute at STP), this carrier gas being the same as the buffer gas. The carrier gas flow rate and the pressure in the oven are controlled and adjusted in order to change the anthracene flow rate qant coming out of the oven. At the temperature of the oven, qant is determined from the carrier flow rate qcarrier, the anthracene vapor pressure Pvap, and the total pressure in the oven Poven by the relation
qant ) qcarrier
Pvap(T) Poven - Pvap(T)
(1)
To avoid blocking the oven exit and the line by large-sized solid particles of anthracene, the gaseous mixture is filtered by a Swagelock 0.5 mm filter (Swagelok SS-4FWS-05) inside the
3134 J. Phys. Chem. A, Vol. 110, No. 9, 2006 oven. To avoid condensation downstream of the oven, the gas line between the oven and the reservoir is heated at a higher temperature than the oven, via a 600-W heating wire wound around the tube. The remaining parts of the experimental apparatus are at room temperature. When the cold supersonic jet meets the surface chamber, which is in front of the nozzle, it becomes subsonic and recovers the initial temperature value, that is, the temperature of the reservoir. Therefore, this surface is cooled by chilled water (15 °C) to maintain a temperature low enough to protect the Viton O-rings. In addition, to avoid too much anthracene penetrating into the pumps, a big cooled collector has been placed between the chamber and the pumps. The CH radical is produced through the photolysis of Bromoform (CHBr3) introduced in the reservoir. The precursor, liquid at room temperature, is placed in a glass vessel outside the CRESU chamber, and its vapor is entrained by bubbling a small controlled flow of He through it. Photolysis of bromoform is achieved using the fourth harmonic radiation (266 nm) of a Nd:YAG pulsed laser (Spectra Physics, GCR 190, 10 Hz, 20 mJ) propagated along the flow. A sonic expansion of helium through a 10-mm hole is used to avoid anthracene deposition in the photolysis entrance window, which would decrease the photolysis laser energy going into the main chamber. The 266nm laser radiation produces a stream tube filled with a uniform density of CH radicals. The successive absorption of three photons necessary to dissociate CHBr3 at the wavelength of 266 nm yields a non-negligible fraction of vibrationally excited CH radicals. To ensure full thermalization of the CH radicals (only the ground state is probed), it is therefore necessary to select a carrier gas which efficiently relaxes CH(V > 0). Earlier kinetic studies33 have shown that nitrogen (N2)sin contrast to the commonly used helium gas for instancesmeets this requirement. The experimental rate coefficient for the reaction
Goulay et al.
Figure 2. First-order decay of LIF signal from CH in the presence of 47.7 × 1012 molec cm-3 of anthracene at 119 K in nitrogen, fit to a single-exponential decay, with residuals shown above.
CH(V ) 1) + N2 f CH(V ) 0) + N2 drops from 10-10 cm3 s-1 at 23 K to almost 10-11 cm3 s-1 at 584 K. For total number densities in the (2-4) × 1016 cm-3 range, the corresponding reaction times are ∼0.25 and 5 µs for low and high temperatures, respectively. These values remain small compared to the hydrodynamic time of the gas flow (>100 µs). For experiments performed in helium as buffer gas (a flow temperature of 58 K), a small amount of nitrogen ( 1). After formation the CH radicals travel downstream and react with the species present in the flow. Once in the detection region (∼200 mm after the nozzle exit), CH radicals are probed by laser-induced fluorescence (LIF) exciting the CH(A2∆, V ) 0 r X2Π, V ) 0) transition at 431 nm with a pulsed MOPO tunable laser (Spectra Physics MOPO 730, with frequency doubling option, 10 Hz, ∼10 µJ) or a pulsed tunable dye laser (Spectron, 10 Hz,
